Metal blades are formed with an airfoil section for interface with a working fluid and a root section for attachment of the blade to a rotating metal disk. The root section may have any of a variety of known shapes (for example, dovetail, T-root, Christmas or fir tree, or a threaded attachment), but in each instance, the root section conforms to a corresponding attachment shape on the disk with a desired degree of looseness to facilitate the installation and removal of the blade. Slight movement between a metal blade root section and the corresponding attachment geometry on the disk during operation of the apparatus is accommodated by the inherent fretting resistance of the blade and disk materials.
Composite blades (for example epoxy, glass or ceramic materials) have advantages over metal blades, particularly their higher strength to weight ratio which allows for the design of lower weight parts. Composite blades have been used in compressors, fans, propellers and turbines. However, composite materials are not inherently fret-resistant, and the attachment of a composite blade to a metal disk is problematic. Special attachment techniques have been developed to address this problem, such as the use of a compliant interface layer and specially designed joints. Alternatively, composite disks have been designed with integrally formed composite blades. Nonetheless, improved attachment designs for composite blades are still desired.